In advanced technology nodes of integrated circuit industry, high k dielectric material and metal are adopted to form a metal gate stack of metal-oxide-semiconductor field-effect transistors (MOSFETs). In a method to form a metal gate stack, a photolithography process is implemented for metal gate patterning. A patterned photoresist layer is formed on the gate material layers, an etching process is then performed to the gate material layers, and then the patterned photoresist layer is removed thereafter by an oxygen ashing process (O2 plasma). However, the O2 plasma removal will cause metal layer oxidation and initial oxide layer re-growth. Furthermore, polymer removal or polysilicon/metal/high k sidewall treatment after gate etching will also cause metal gate oxidation, oxygen penetration to the high k dielectric material layer, and initial oxide layer re-growth.